This invention relates generally to a sootblower device for directing a fluid spray against a heat exchanger surface and particularly to such a device providing improvements in the uniformity of the cleaning effect provided.
Cleaning highly heated surfaces, such as the heat exchange surfaces of a boiler, furnace or the like, has commonly been performed by devices generally known as sootblowers. Sootblowers typically employ water, steam, air, or a combination thereof, as a blowing medium which is directed through a nozzle against encrustations of slag, ash, scale, and/or other fouling materials which become deposited on the heat exchange surfaces. Sootblowers of the retracting variety employ a lance tube which is advanced into the boiler through a wall port and have one or more nozzles through which the cleaning medium is discharged.
It has long been known that water in liquid form, either used alone or in combination with a gaseous blowing medium, increases the ease with which the encrustations are dislodged. The effectiveness of water in dislodging the encrustations results from a thermal shock effect coupled with mechanical impact. The thermal shock shrinks and embriddles the encrustations resulting in a fracturing of the encrustations so that they become dislodged and fall away because of the mechanical impact.
Unfortunately, to obtain sufficient cleaning in accordance with the water spray process mentioned above, the danger of overstressing the hot surfaces is present. In fact, rapid deterioration of the heat exchange surfaces as a result of the thermal shock has been seen. The problem of heat exchange surface deterioration has been particularly severe in connection with cleaning the rigidly held tube bundles of large scale boilers. Being rigidly held, the tubes cannot readily distort in response to the temperature induced shrinkage and expansion occurring during a cleaning cycle. Difficulties are present in an effort to produce adequate cleaning performance while avoiding thermal overstressing since the surfaces to be cleaned are of varying distances from the nozzle and therefore a varying speed of jet progression across the heat exchanger surfaces occurs. Areas of slow progression may receive excessive thermal shock whereas areas of fast progression may not be provided an adequate cleaning effect.
Another significant consideration in sootblower operations is the cost effectiveness of operation. Sootblowers have a significant power requirement for operation, use a large quantity of cleaning medium, and place a thermal load on the boiler.
One method previously used in an attempt to control the induced thermal shock and provide for efficient sootblower operation, involved throttling the blowing medium. The blowing medium was throttled in a manner such that the amount of blowing medium striking the different surfaces of the heat exchanger would remain substantially constant. For example, when cleaning the wall through which a retractable sootblower lance is inserted and rotated, the jet of the blowing medium, when projected back toward the wall, traces an increasing diameter spiral path as the extended length of the lance increases. To maintain the amount of blowing medium striking the different surfaces of the wall substantially the same, the flow rate of the blowing medium can be reduced when the blowing medium is discharged against surfaces close to the lance and increased as the blowing medium was discharged against surfaces farther away from the lance. By varying the rate of discharge, the total quantity of blowing medium striking incremental areas can be maintained constant. This approach, however, has the disadvantage that the jet velocity decreases with flow rate through the lance nozzle. This decreased jet velocity has been found to degrade cleaning performance by reducing the mechanical impact force, which coupled with thermal shock, cleans the heat exchange surfaces.
Another approach toward providing a more uniform cleaning effect is to provide a control mechanism for a wall blower that varies the rotational and translational speed of the lance during the cleaning cycle of a retractable sootblower. If a constant motor speed is utilized, the angular rate of rotation will be constant and the rate of travel of the jet's impingement point, with respect to the wall surface, will be slower in the smaller diameters of the spiral, e.g. where the nozzle is close to the wall, and will be fastest where the spiral diameters are greatest and the lance is near full insertion. By decreasing the speed of the motor, it is possible to maintain relatively constant jet progression over the entire course of the spiral by decreasing the rate of rotation of the lance as the jet impinges against the wall in those spiral areas of larger diameter. Thus, the rate of rotation of the lance is solely a function of the extension distance of the lance or nozzle into the boiler. The assignees of this invention, the Babcock & Wilcox Company, have been granted U.S. Pat. No. 3,782,336 (and reissue thereof Re. 32,517) encompassing such a wall cleaning sootblower.
A drawback of the above approach is that it is limited in its applicability. The method works satisfactorily for a wall blower where the surface to be cleaned is generally perpendicular to the insertion axis of the lance. However, the result is unsatisfactory when the surfaces to be cleaned are not oriented such that the jet progression rate is a simple function of the lance extension distance.